Electronic circuit device and method of making the same

ABSTRACT

A method of making an electronic circuit device includes placing a circuit board in a cavity of a mold such that one side of the circuit board is held in close contact with an inner surface of the cavity, and encapsulating the circuit board in a casing by filling the cavity with a resin material. The one side of the circuit board is exposed to one side of an outer surface of the casing to define part of the one side of the outer surface of the casing. The method further includes thinning the casing by machining the entire one side of the outer surface of the casing.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.12/243,217, filed Oct. 1, 2008, which claims priority to JapaneseApplication No. 2007-257836, filed Oct. 1, 2007, the disclosures of bothof which are incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to an electronic circuit device and methodof making the electronic circuit device.

BACKGROUND OF THE INVENTION

U.S. 2007/0161269 corresponding to JP-A-2006-303327 discloses anelectronic circuit device configured as an electronic key transceiver.The electronic circuit device includes a circuit board having first andsecond sides opposite each other. Electronic elements are mounted ononly the first side of the circuit board. The circuit board isencapsulated in a casing in such a manner that the second side of thecircuit board is exposed to an outer surface of the casing. Thus, thesecond side of the circuit board defines part of the outer surface ofthe casing.

A method of making the electronic circuit device includes a placingprocess and an encapsulating process subsequent to the placing process.In the placing process, the circuit board is placed in a cavity of amold (i.e., die) in such a manner that the second side of the circuitboard can be held in close contact with an inner surface of the cavity.In the encapsulating process, a liquid resin material is injected underpressure into the cavity of the mold and then cured (i.e., solidified).

Since the circuit board is held in close contact with the inner surfaceof the cavity, the circuit board can be prevented from being greatlydeformed by pressure and heat, which are applied to the circuit board inthe encapsulating process.

In the method described above, after the resin material is injected tothe cavity, air may be trapped in a gap between the electronic elementand the circuit board. The air trapped in the gap may expand due to thepressure and heat and cause a bulge (bump or the like) on the secondside of the circuit board. Further, the second side of the circuit boardmay jut out due to a difference in coefficients of linear (thermal)expansion between the circuit board and the casing. Since the secondside of the circuit board is exposed to the outer surface of the casing,the bulge and the jut mar the appearance of the electronic keytransceiver.

Such a deformation on an exposed side of the circuit board can beeliminated by a sanding technique or the like. However, generally, thedeformation on the exposed side of the circuit board is very small, hasa complex shape, and differs from product to product. Therefore, it isdifficult to automate an eliminating process to eliminate thedeformation, and the eliminating process may be performed manually. As aresult, it takes a lot of man hours to eliminate the deformation.Further, the manual eliminating process may result in a decrease inaccuracy (i.e., flatness) of the second side of the circuit board.

SUMMARY OF THE INVENTION

In view of the above-described problem, it is an object of the presentinvention to provide an electronic circuit device and a method of makingthe electronic circuit device to allow automation of an eliminatingprocess to eliminate a deformation on an exposed surface of a circuitboard.

According to an aspect of the present invention, a method of making anelectronic circuit device includes preparing a circuit board and anelectronic element mounted on only a first side of the circuit board,placing the circuit board in a cavity of a mold such that a second sideof the circuit board is held in close contact with an inner surface ofthe cavity. The first and second sides of the circuit board are oppositeeach other. The method further includes encapsulating the circuit boardand the electronic element in a casing by filling the cavity with aresin material. The casing has a first thickness in a direction from thefirst side to the second side of the circuit board. The method furtherincludes removing the casing from the mold. The second side of thecircuit board is exposed to one side of an outer surface of the casingto define part of the one side of the outer surface of the casing. Themethod further includes thinning the casing from the first thickness toa second thickness by machining the entire one side of the outer surfaceof the casing.

According to another aspect of the present invention, an electroniccircuit device includes a circuit board having first and second sidesopposite each other, an electronic element mounted on only the firstside of the circuit board, and a resin casing configured to encapsulatethe circuit board and the electronic element. The second side of thecircuit board is exposed to one side of an outer surface of the casingto define part of the one side of the outer surface of the casing. Theentire one side of the outer surface of the casing is machined.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with check to the accompanying drawings. In thedrawings:

FIG. 1 is a diagram illustrating a top view of a circuit board of anelectronic key transceiver according to an embodiment of the presentinvention;

FIG. 2 is a diagram illustrating a cross-sectional view of a placingprocess of a method of making the electronic key transceiver;

FIG. 3 is a diagram illustrating a cross-sectional view of anencapsulating process of the method of making the electronic keytransceiver;

FIG. 4 is a diagram illustrating a cross-sectional view of a removingprocess of the method of making the electronic key transceiver;

FIG. 5A is a diagram illustrating a cross-sectional view of asemi-finished electronic key transceiver, and FIG. 5B is a diagramillustrating a cross-sectional view of the electronic key transceiver asa finished product;

FIG. 6 is a diagram illustrating a cross-sectional view of thesemi-finished electronic key transceiver having a warpage;

FIG. 7 is a diagram illustrating a cross-sectional view of thesemi-finished electronic key transceiver having a bulge; and

FIG. 8A is a diagram illustrating a cross-sectional view of a circuitboard of the semi-finished electronic key transceiver, and FIG. 8B is adiagram illustrating a cross-sectional view of the circuit board of theelectronic key transceiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electronic key transceiver 1 according to an embodiment of thepresent invention is described below with reference to FIGS. 1-5B. FIG.5B illustrates the electronic key transceiver 1 as a finished product.The electronic key transceiver 1 includes a circuit board 2, anelectronic element 3 mounted on the circuit board 2, and a casing 4 madeof a resin material. For example, the electronic key transceiver 1 canbe used in a vehicle electronic key system and carried by a vehicle'sdriver.

As illustrated in detail in FIG. 1, positive and negative terminals 5, 6are soldered to the circuit board 2. The circuit board 2, the electronicelement 3, and solder joints between each of the positive and negativeterminals 5, 6 and the circuit board 2 are encapsulated in the casing 4.

The circuit board 2 can be made, for example, by forming an electricalconductor trace pattern (e.g., copper foil) to an electrical insulatorbase such as a glass epoxy board. In the embodiment, the circuit board 2employs a glass-reinforced epoxy board as the electrical insulator base.Alternatively, the electrical insulator base can be a board other than aglass-reinforced epoxy board.

The circuit board 2 has first and second sides 21, 22 opposite eachother. The electronic element 3 is mounted on only the first side 21 ofthe circuit board 2 so that the second side 22 of the circuit board 2can be flat. An example of the electronic element 3 can be a resistor, acapacitor, a diode, a transistor, an integrated circuit (IC) module, anantenna, or the like.

The circuit board 2 has a cutout 23 that defines a battery space where abattery (not shown) is accommodated. For example, the battery can be abutton-type battery. The positive terminal 5 spans the cutout 23 and issoldered to the trace pattern of the circuit board 2 at each end.Likewise, the negative terminal 6 spans the cutout 23 and is soldered tothe trace pattern of the circuit board 2 at each end. When the batteryis accommodated in the battery space, positive and negative poles of thebattery are in contact with the positive and negative terminals 5, 6,respectively. Thus, the electronic key transceiver 1 can be powered bythe battery accommodated in the battery space.

The casing 4 of the electronic key transceiver 1 is made by using a mold100 (i.e., a die) illustrated in FIG. 2. The circuit board 2, to whichthe electronic element 3 and the positive and negative terminals 5, 6are mounded, is placed in a cavity 104 of the mold 100 in such a mannerthat the second side 22 of the circuit board 2 can be held in closecontact with an inner surface of the cavity 104.

The mold 100 includes an upper mold 101, a lower mold 102, and a slidecore (not shown). The slide core is sometimes also referred as a “sidecore”. The slide core covers middle portions of the positive andnegative terminals 5, 6 to form the battery space. The upper and lowermolds 101, 102 are fixed to movable or fixed platens of a moldingmachine (not shown).

The upper mold 101 of the mold 100 has a sprue (i.e., runner) 107. A pot105 is formed on an upstream side of the sprue 107, and a gate 108 isformed on a downstream side of the sprue 107. A plunger 106 (i.e.,piston) of the molding machine is located above the pot 105 so that theplunger 106 can enter and exit the pot 105. A tablet 110 as a solidresin material is charged into the pot 105, and then the plunger 106enters the pot 105. As a result, the tablet 110 can change to a liquidresin material. The liquid resin material flows from the pot 105 to thecavity 104 by passing through the sprue 107 and the gate 108.

The lower mold 102 of the mold 100 has a suction hole 109 exposed to asurface 102 a. The suction hole 109 is coupled through a pipe (notshown) to an external suction source (not shown) such as a vacuum pump.

A method of making the electronic key transceiver 1 is described belowwith reference to FIGS. 2-5A. The method includes a placing processillustrated in FIG. 2, an encapsulating process subsequent to theplacing process and illustrated in FIG. 3, a removing process subsequentto the encapsulating process and illustrated in FIG. 4, and a thinningprocess subsequent to the removing process and illustrated in FIG. 5A.

Firstly, the placing process is described below with reference to FIG.2. In the placing process, the second side 22 of the circuit board 2 isplaced on the surface 102 a of the lower mold 102 so that the circuitboard 2 can be located on the suction hole 109. Then, the upper mold 101and the lower mold 102 are assembled and clamped together to form thecavity 104. As a result, the circuit board 2 is located in the cavity104 of the mold 100. At this time, the suction hole 109 is maintained atnegative pressure by the suction source with respect to the cavity 104so that the circuit board 2 can be held in close contact with the innersurface of the cavity 104.

Then, the tablet 110 as the resin material for the casing 4 is chargedinto the pot 105 of the mold 100. The tablet 110 can be made ofthermoset resin. In the embodiment, the tablet 110 is made of epoxyresin. For example, B-stage (i.e., semi-cured) epoxy resin powder iscompressed into the tablet 110. The use of the tablet 110 (i.e., the useof solid material instead of liquid material) can improvemanufacturability of the electronic key transceiver 1 and also helpprevent air bubble from being trapped in the casing 4 of the electronickey transceiver 1. If necessary, the tablet 110 can be preheated beforebeing charged into the pot 105.

A temperature of the mold 100 needs to be adjusted to cause a curingreaction of the epoxy resin. Further, since the electronic element 3 andthe positive and negative terminals 5, 6 are joined to the circuit board2 by solder, the temperature of the mold 100 needs to be less than amelting temperature of the solder. In the embodiment, the solder has amelting temperature of about 240° C., and the epoxy resin has a curingreaction temperature of about 170° C. Therefore, for example, thetemperature of the mold 100 can be adjusted to about 200° C.

As describe above, the casing 4 of the electronic key transceiver 1 ismade of the epoxy resin. Since the epoxy resin has high heat resistanceand high mechanical resistance, the electronic key transceiver 1 can besuitably protected against damage. Therefore, although the electronickey transceiver 1 is carried by the driver at all times, a reliabilityof the electronic key transceiver 1 can be ensured.

The resin material for the casing 4 of the electronic key transceiver 1can be thermoset resin other than epoxy resin, as long as a curingreaction temperature of the resin material is less than a meltingtemperature of the solder. For example, the resin material can bephenolic resin, unsaturated polyester resin, or the like.

Next, the encapsulating process performed subsequent to the placingprocess is described below with reference to FIG. 3. In theencapsulating process, the plunger 106 moves down and enters the pot 105so that the tablet 110 located in the pot 105 can change to the liquidepoxy resin. The liquid epoxy resin is injected into the cavity 104through the sprue 107 and the gate 108 so that the cavity 104 can befilled with the liquid epoxy resin. The liquid epoxy resin in the cavity104 receives heat from the mold 100. The heat causes a curing reactionof the liquid epoxy resin. As a result, the liquid epoxy resin is curedand molded into the casing 4. In this way, the circuit board 2 isencapsulated in the casing 4 made of the resin material.

Next, the removing process performed subsequent to the encapsulatingprocess is described below with reference to FIG. 4. In the removingprocess, the mold 100 is opened, and the circuit board 2 encapsulated inthe casing 4 is removed from the mold 100 by using an eject mechanism(not shown) of the molding machine. Then, as shown in FIG. 5A, thecasing 4 is cut along the gate 108 to eliminate an unnecessary portioncorresponding to the pot 105 and the sprue 107. In this way, asemi-finished electronic key transceiver 1A as a semi-finished productcan be made.

The casing 4 encapsulates the electronic element 3 mounted on thecircuit board 2, the first side 21 and a side surface 24 of the circuitboard 2, and the solder joints between each of the positive and negativeterminals 5, 6 and the circuit board 2. Therefore, circuitry of theelectronic key transceiver 1 can be sealed in the casing 4 so that theelectronic key transceiver 1 can be made waterproof.

The second side 22 of the circuit board 2 is exposed to one side of anouter surface of the casing 4 and forms a flat continuous surface inconjunction with the one side of the outer surface of the casing 4.Thus, the second side 22 of the circuit board 2 defines part of the oneside of the outer surface of the casing 4.

The electronic key transceiver 1 has a rectangular shape like a card.For example, the electronic key transceiver 1 can be sized tosubstantially meet an ID-1 format (85.60×53.98 mm) specified by ISO7810, except for the thickness.

As can be seen by comparing FIGS. 5A and 5B, a thickness of thesemi-finished electronic key transceiver 1A is slightly greater than athickness T1 of the electronic key transceiver 1.

Specifically, the one side of the outer surface of the casing 4 of thesemi-finished electronic key transceiver 1A extends beyond a target linecorresponding to the thickness T1 by a predetermined distance. Thetarget line is represented by a two-dot chain line in FIGS. 5A, 5B.

In the embodiment, the distance is set to about 0.3 millimeters. Thatis, the thickness of the semi-finished electronic key transceiver 1A isabout 0.3 millimeters greater than the thickness T1 of the electronickey transceiver 1.

When the distance is equal to or greater than about 0.2 millimeters, thethinning process described later can be accurately performed.Specifically, the entire one side of the outer surface of the casing 4of the semi-finished electronic key transceiver 1A can be uniformlymachined. However, when the distance exceeds 0.4 millimeters, the amountof wasted material increases. Therefore, it is preferable that thedistance be from 0.2 millimeters to 0.4 millimeters.

Finally, the thinning process performed subsequent to the removingprocess is described below with reference to FIGS. 5A, 5B. In thinningprocess, the semi-finished electronic key transceiver 1A shown in FIG.5A is thinned down to the thickness T1. Specifically, the one side ofthe outer surface of the casing 4 of the semi-finished electronic keytransceiver 1A is machined (i.e., grinded, polished, or the like) forexample, using a sanding machine, until the target line is exposed. Inthis way, the electronic key transceiver 1 shown in FIG. 5B is made. Ifnecessary, a surface treatment such as coating (painting) is applied toan outer surface of the electronic key transceiver 1 so that themachined side of the electronic key transceiver 1 can have a goodappearance.

After the encapsulating process is finished, the casing 4 of thesemi-finished electronic key transceiver 1A may contract (i.e., shrink)with a decrease in temperature. As a result, as shown in FIG. 6, thesemi-finished electronic key transceiver 1A may warp such that thesecond side 22 of the circuit board 2 juts out.

As shown in FIG. 7, after the encapsulating process is finished, air maybe trapped in a gap between the circuit board 2 and the electronicelement 3. The air trapped in the gap may cause a bulge (bump or thelike) on the second side 22 of the circuit board 2 of the semi-finishedelectronic key transceiver 1A.

As mentioned previously, the second side 22 of the circuit board 2 isexposed to the one side of the outer surface of the casing 4 to definepart of the one side of the outer surface of the casing 4. In thethinning process, the entire one side of the outer surface of the casing4 of the semi-finished electronic key transceiver 1A is uniformlymachined so that the warpage and bulge on the second side 22 of thecircuit board 2 can be surely eliminated. Therefore, the electronic keytransceiver 1 as a final product can have no warpage and no bulge.

For example, the electronic key transceiver 1 is kept in a wallet, whenthe driver carries the electronic key transceiver 1. Therefore, it ispreferable that the thickness T1 of the electronic key transceiver 1 beless than or equal to about 3 millimeters so that the electronic keytransceiver 1 can be put in the wallet.

In some applications, a thickness T2 of the electronic element 3 mountedon the circuit board 2 can reach up to about 1.5 millimeters. It ispreferable that a thickness T3 of the casing 4 located on the electronicelement 3 be greater than or equal to about 0.5 millimeters so that theelectronic element 3 can be surely protected by the casing 4. Therefore,a thickness T of the base of the circuit board 2 of the electronic keytransceiver 1 needs to be less than 1 millimeter (i.e., T=T1-T2-T3).

FIG. 8A illustrates the circuit board 2 of the semi-finished electronickey transceiver 1A. FIG. 8B illustrates the circuit board 2 of theelectronic key transceiver 1. That is, FIG. 8A illustrates the circuitboard 2 before the thinning process is finished, and FIG. 8B illustratesthe circuit board 2 after the thinning process is finished.

As shown in detail in FIG. 8A, the circuit board 2 includes a first base34, a prepreg layer 35, and a second base 36. Trace patterns 341, 342are formed on both sides of the first base 34, respectively. The tracepatterns 341, 342 are electrically connected together via a through hole(not shown) in the first base 34. The second base 36 has no tracepattern. The prepreg layer 35 is sandwiched between the first and secondbases 34, 36 and serves as a adhesive layer. The first base 34, theprepreg layer 35, and the second base 36 are joined together by hotpressing.

In FIG. 8B, a distance from the second side 22 of the trace pattern 342needs to be greater than about 0.4 millimeters in order to ensure aresistance to static electricity. Further, a glass epoxy board, which isused for the first base 34, having a thickness of less than about 0.2millimeters is less commonly available in a market. Therefore, it ispreferable that the thickness T of the base of the circuit board 2 ofthe electronic key transceiver 1 be greater than or equal to about 0.6millimeters. That, it is preferable that the thickness T of the base ofthe circuit board 2 of the electronic key transceiver 1 be from about0.6 millimeters to about 1 millimeter.

In the embodiment, the first base 34 has a thickness of about 0.2millimeters, the second base 36 has a thickness of about 0.7millimeters, and the prepreg layer 35 has a thickness of about 0.2millimeters. Therefore, the thickness of the base of the circuit board 2of the semi-finished electronic key transceiver 1A is about 1.1millimeters. In the thinning process, the second base 36 of thesemi-finished electronic key transceiver 1A is thinned by about 0.3millimeters so that a second base 36A of the electronic key transceiver1 can have a thickness of about 0.4 millimeters. Therefore, thethickness T of the base of the circuit board 2 of the electronic keytransceiver 1 is about 0.8 millimeters.

As described above, according to the embodiment, the semi-finishedelectronic key transceiver 1A is thicker than the electronic keytransceiver 1 as a finished product. The semi-finished electronic keytransceiver 1A is thinned by uniformly machining the entire one side ofthe outer surface of the casing 4 in the thinning process. Accordingly,the second side 22 of the circuit board 2 is uniformly machined so thata deformation (i.e., bulge, warpage, or the like) on the second side 22of the circuit board 2 can be surely eliminated.

In the thinning process, the one side of the outer surface of the casing4 is entirely machined, regardless of size or shape of the deformationon the second side 22 of the circuit board 2. Therefore, the thinningprocess can be automated.

The second side 22 of the circuit board 2 is exposed to the one side ofthe outer surface the casing to define part of the one side of the outersurface of the casing 4. In such an approach, the thickness T1 of theelectronic key transceiver 1 can be reduced.

The circuit board 2 employs the glass-reinforced epoxy board as thebase. In such an approach, the electronic key transceiver 1 can haveboth stiffness and toughness.

In the thinning process, the one side of the outer surface of the casing4 is machined. Therefore, although the base of the circuit board 2 is aglass-reinforced board, the deformation on the second side 22 of thecircuit board 2 can be accurately, easily removed.

The embodiment described above can be modified in various ways. Forexample, the thinning process can include grinding, polishing, orcutting the one side of the outer surface of the casing 4. For example,the one side of the outer surface of the casing 4 can be graduallypolished using a polishing compound, until the target line is exposed.Alternatively, the one side of the outer surface of the casing 4 can becut along the target line.

The casing 4 can be made by using a molding technique other than atransfer molding technique. For example, the casing 4 can be made byusing an injection molding technique, a compression molding technique,or the like.

The electronic key transceiver 1 (i.e., the casing 4) can have a shapeother than a card. The present invention can be applied to an electroniccircuit device other than an electronic key transceiver.

Such changes and modifications are to be understood as being within thescope of the present invention as defined by the appended claims.

1. A method of making an electronic circuit device comprising: preparinga circuit board and an electronic element mounted on only a first sideof the circuit board; placing the circuit board in a cavity of a moldsuch that a second side of the circuit board is held in close contactwith an inner surface of the cavity, the second side being opposite tothe first side; and encapsulating the circuit board and the electronicelement in a casing by filling the cavity with a resin material, thecasing having a first thickness in a direction from the first side tothe second side of the circuit board; removing the casing from the mold,the second side of the circuit board being exposed to one side of anouter surface of the casing to define part of the one side of the outersurface of the casing, and thinning the casing from the first thicknessto a second thickness by machining the entire one side of the outersurface of the casing.
 2. The method according to claim 1, wherein thecasing has a substantially card shape.
 3. The method according to claim2, wherein the circuit board includes a base made of glass-reinforcedepoxy resin.
 4. The method according to claim 3, wherein the thinning ofthe casing includes polishing the entire one side of the outer surfaceof the casing.